1. Field of the Invention
This invention relates to hydraulic cement compositions. It further relates to hydraulic cement compositions which include an additive to retard the rate of hardening of a water slurry of a hydraulic cement composition. Such an additive is referred to in the art as a set retarder. This invention also relates to a method of delaying the set of a water slurry of a hydraulic cement composition. This invention still further relates to a method of cementing a subterranean formation penetrated by a wellbore.
2. Description of the Prior Art
Hydraulic cements, i.e., inorganic cements that harden or set under water, are customarily admixed with water and emplaced as aqueous slurries. The amount of water employed may vary rather extensively dependent largely upon the fluidity of the slurry required and upon the necessary ultimate strength. Additives may be employed to modify the properties of the basic cement slurry.
Ordinary Portland cement, when mixed with water and subjected to elevated temperatures, begins to hydrate immediately and sets in a short time to an extent that its ability to be pumped over a long period of time is limited.
In cementing operations, particularly in moderate and high temperature environments, such as may be encountered in well cementing, it is often necessary, or at least highly desirable, to retard the rate at which a cement slurry would normally set in order to provide a thickening time of sufficient duration to permit placement of the cement slurry where desired. It is a prime requisite of cements that they have the property of remaining fluid and pumpable for a period of time at elevated temperatures to permit placement before setting begins.
Cement compositions having retarded setting rates are useful for a number of purposes including the placement of them in boreholes which penetrate subterranean earth formations, such as wells. In well cementing practice, Portland cement is normally mixed with water, based on the weight of the dry cement, to form a slurry which is pumped into the well to the desired point behind the casing where it is allowed to harden. Inasmuch as wells are frequently of great depth, extended times are required to mix the cement slurry and pump it to the desired point. It is important, therefore, that the cement employed have a thickening time long enough to allow placement of the cement slurry in the desired location in the well before it becomes stiff and unpumpable. Experience has shown that the elevated pressures and temperatures encountered in cementing wellbores which penetrate subterranean formations accelerate the normal setting rate of cement to the point that the placement time of the cement approaches or exceeds the thickening time of the cement thus rendering difficult or impossible the placement of the cement slurry in the desired location in the well. Successful placement of the slurry, therefore, requires that the slurry remain fluid and pumpable at high temperatures for several hours before it begins to harden. However, after the slurry has been pumped into place, it is desirable that hydration or set proceed at a rate at which the slurry will attain its final set and develop considerable strength within about 24 hours.
Various methods of retarding the setting rates of cements have been employed in order to make possible the placement of cement in wells. In general, cements for use in wells are coarsely ground materials prepared by (1) altering the composition of the cement itself, (2) adding various special retarding agents to the cement, or (3) a combination of methods 1 and 2. Set retarding agents which have been employed include casein, sodium carboxymethyl cellulose, carboxymethyl hydroxyethyl cellulose, dextrin, starch, compounds of boron, bicarbonates and tartrates, water soluble gums, lignosulfonates, copolymers of acrylic acid and 2-acrylamido, 2-methylpropane sulfonic acid and combinations of these and other materials.